Disk drive systems utilizing closed-looped head positioning control systems rely on servo data stored on the rotating disks as the source of data track positioning feedback information. One approach to providing such servo information is to dedicate an entire disk surface and corresponding servo read data channel for the near-continuous sourcing and capture of positioning information. However, dedication of an entire disk surface and the need to provide special servo control read circuitry results in a significant increase in the cost per unit data of the disk drive control system and disk drive as a whole.
An alternate approach is to provide servo information within each track sector on every data surface of the disk drive system, i.e., embedded servo. Conventionally, A/B servo bursts of constant frequency and amplitude servo information are recorded as sequential fields within the sector header of each data sector. The servo burst fields are written symmetrically offset from and on respective sides of the data track center line by at least one-half of the head width. That is, the servo bursts do not overlap the track center line. Consequently, the differences between the relative voltage amplitudes, (V.sub.A -V.sub.B), as read by the head while track following, may be utilized as a direct indication of the distance and direction of the head from the track center line.
There are, however, a number of drawbacks to providing servo information in each sector header. Since the servo information is in-line with the sector data, the percentage of sector length available for storing user data is reduced. The aggregate reduction in user data storage space is often many megabytes, if not tens of megabytes, as a consequence of using embedded servo information.
Another problem with conventional embedded servo systems is that the servo information is received only once per sector. Each sector, however, follows a uniform arc proportional to its radial distance from disk center. There are non-linear forces acting on the actuator arm and head that require active compensation. These forces include shock and vibration, and to a lesser extent, the air drag of the arm and head and the torquing force of the flex circuit. However, such forces and any error in the previous position correction will result in head drift that remains uncompensated until the next sector's servo burst can be obtained and a new correction applied.
Head drift during a sector read operation will result in data errors if the drift is significant. Conversely, excessive drift during a write operation is unacceptable due to the compromise of data stored on adjacent tracks. Therefore, track spacing is predominantly limited by the accuracy or tightness of the servo control loop, that is, in turn, dependent on sector length and disk rotation rate, i.e., the effective frequency of occurrence of servo bursts with respect to the read head.
Another limitation of A/B servo burst control systems arises from the extent of off-track error that can be unambiguously determined based on the servo burst fields. Whether as the end condition of a seek operation or the consequence of vibration or mechanical impact, a read head may be at a position substantially offset from the desired track center line. Beyond an off-track range limit from track center line, the servo control loop will fail to produce a proper position correction as a consequence of reading servo burst data from an adjacent track. The servo control loop will therefore issue an inaccurate position correction or incorrectly select the then closest track center line as being the desired track center line for track following. A time-consuming additional seek operation is then required to change tracks. Typically the off-track capture range of A/B burst servo control systems is no more than about .+-.3/8 of a track width.
The conventional A/B servo burst pattern itself may give rise to loss of tracking accuracy as a consequence of incorrect placement of the servo burst fields. Track formatting, or servo track writing, is done to provide the servo burst fields as part of the initial drive fabrication. Although done under the electromechanical control of a precision servo writer controller, variances due to vibration, out of round spindle bearings, and nonlinearities in the servo writer's own mechanics will result in one or more servo bursts being written offset from their ideal location. Surface defects can also distort the shape and effective position of servo bursts. If left uncorrected, subsequent track following operations will follow an improper track center line or slip entirely off track. For conventional A/B burst patterns, the off-track error is equal to the net servo burst field offset error. Typically, data sectors having incorrectly written servo bursts are marked as simply unusable.
Another concern relevant to the use of embedded servo is the consequence of increased track density. Conventional A/B burst patterns are written by a servo pattern writer using a series, typically four, of write head passes. This results in a series of overlap erasures of the partial patterns written during prior passes. The erasure is a consequence of a fringe electromagnetic field extending beyond the head element when the head is energized to write. The problem is that, as the track density is increased, the proportion of the servo bursts that are fringe field erased increases. That is, the total width of the bursts perpendicular to the track center line is decreased, but the width of the erasure is constant. Consequently, there is a loss of track following accuracy due to the decreased ability to discriminate track offset errors based on the decreased recoverable signal strength of the A/B servo bursts.
Quad-burst servo field patterns have been proposed for use in embedded servo systems. U.S. Pat. No. 4,669,004 discloses a quad-burst pattern where one of four sequential servo fields is disposed symmetrically across the track center line, two others are symmetrically offset from the track center line, and a fourth field is asymmetrically disposed well above the track center line. This burst pattern identically repeats every fourth track at the same sector.
The individual burst offsets are related as integer multiples of less than or equal to one-half of the head width. Each servo field is identically provided with absolute track identification information as well as a track center line relative burst field. The significance of providing four such servo fields, as opposed to just two servo bursts, is that all modes of track-seeking, up to a preset maximum seek rate, will result in the read head crossing substantially over at least one of the servo fields.
The quad-burst servo pattern disclosed in U.S. Pat. No. 4,669,004, however, is also sensitive to servo burst offset errors. Although four servo bursts are present, only two of the bursts are used in determining the relative off-track center line distance and direction. These two bursts are functionally and positionally equivalent to the A/B bursts. Accordingly, this quad-burst servo system obtains no better insensitivity to servo burst position offset errors than the conventional two-burst servo pattern systems. Further, another limitation of the quad-burst servo pattern of U.S. Pat. No. 4,669,004 is that it appears to place a constraint on the track pitch based on the width of the read/write head.